Multiple electron mirror apparatus

ABSTRACT

An electron beam system using an electron beam source, a magnetic prism and a plurality of electron mirrors with associated electron beam focusing or cross-section control lenses and beam deflection units permits an electron beam from the electron beam source to be modified or acted upon as required by the electron mirrors as the electron beam is sequentially presented to the electron mirrors. An electron beam is directed to the first electron mirror where it is reflected and directed to the next electron mirror, etc. The electron beam is acted upon at an electron mirror in accordance with the voltage bias applied to the electron mirror. The voltage bias may be set to passively mirror the entire beam or a portion thereof, modulate the beam in accordance with the surface condition of the mirror or permit the beam to impinge on the mirror surface. The electron beam focusing or cross-section control lenses and deflection units provide for selective direction of the beam and control of the cross-section to determine the portion and size of such portion of an electron mirror surface which will influence or be influenced by the electron beam.

ite Maititt States Patent MULTIPLE ELECTRON MIRROR APPARATUS [75]Inventor: Kent N. Mafiitt, Minneapolis, Minn.

[73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul,Minn.

[22] Filed: July 18, 1972 [21] Appl. No.: 272,778

Primary Examiner-Leland A. Sebastian Assistant ExaminerP. A. NelsonAttorney, Agent, or Firm-Alexander, Sell, Steldt and DeLaHunt [451 Feb.4, 1975 ABSTRACT An electron beam system using an electron beam source,a magnetic prism and a plurality of electron mirrors with associatedelectron beam focusing or cross-section control lenses and beamdeflection units permits an electron beam from the electron beam sourceto be modified or acted upon as required by the elsetron mi ror as he etr m is qu tially presented to the electron mirrors. An electron beam isdirected to the first electron mirror where it is reflected and directedto the next electron mirror, etc. The electron beam is acted upon at anelectron mirror in accordance with the voltage bias applied to theelectron mirror. The voltage bias may be set to passively mirror theentire beam or a portion thereof, modulate the beam in accordance withthe surface condition of the mirror or permit the beam to impinge on themirror surface. The electron beam focusing or cross-section controllenses and deflection units provide for selective direction of the beamand control of the cross-section to determine the portion and size ofsuch portion of an electron mirror surface which will influence or beinfluenced by the electron beam.

10 Claims, 5 Drawing Figures sum 10F 2 FIG. 2

PAIENTEDFEB m SHEET 2 OF 2 F IG. 5

1 MULTIPLE ELECTRON MIRROR APPARATUS BACKGROUND OF THE INVENTION 1.Field of the Invention The invention presented herein relates toelectron beam systems and devices and particularly to such systems ordevices using electron mirrors.

2. Discussion of the Prior Art The prior art discloses electron beamelectron mirror systems and devices which use only a single electronmirror together with various types of deflection units, magnetic prismsand beam focusing or cross-section control lenses for controlling theelectron beam. Electron microscopes use a single electron mirror andsystems are known using a single electron mirror where an electron beamis modulated in accordance with the mirror surface condition to which itis presented. US. Pat. Nos. 2,901,627 to Wiskott et al., 3,l76,278 toMayer and 3,278,679 to Newberry are typical of such prior art patents.The prior art does not show or suggest the use of a plurality ofelectron mirrors.

SUMMARY OF THE INVENTION An electron beam electron mirror system isprovided which uses electron mirror techniques and is characterized bythe use ofa plurality of electron mirrors; a magnetic prism; and anelectron beam source for providing an electron beam which is directed atthe magnetic prism which serves to direct the beam toward one of theelectron mirrors where it is acted upon in accordance with the bias onthe electron mirror and returned to the prism, which then directs thebeam to a second one of the plurality of mirrors. This action isrepeated as required at the second mirror and at each of the remainingplurality of electron mirrors. Electron beam focusing or cross-sectioncontrol lenses plus electron beam deflection units are provided for eachof the electron mirrors as may be required to permit the electron beamto be controlled as required during travel to the associated mirror. Thebias voltage presented at a particular mirror determines the action ofthe electron beam at the mirror. The beam may be passively mirrored inits entirety or a portion of the beam may be mirrored, or the beam maybe modulated in accordance with the condition of the mirror surface ormay be permitted to impinge on the mirror surface. The electron beamfocusing or cross-section control lenses and deflection units providefor selective direction of the beam to a portion of the mirror surfaceand control of the cross-section of the beam to establish the size andshape of such portion of an electron mirror which is to influence thebeam or be influenced by the electron beam.

One embodiment of the invention uses a single magnetic prism with theelectron source and the plurality of electron mirrors positioned aboutthe prism. The deflection unit and cross-section control unit when usedwith an electron mirror are positioned between the mirror and themagnetic prism.

Another embodiment uses a single magnetic prism formed from twoelongated, U-shaped magnets, which can be electromagnets or permanentmagnets. The magnets are positioned with a gap between opposite poles ofthe two magnets. The plurality of electron mirrors are positioned aboutthe prism by locating several of the mirrors on each side of themagnetic prism along the length of the prism with the electron beamsource positioned on one side of the magnetic prism. As with the firstembodiment, the deflection unit and crosssection control unit when usedwith an electron mirror are positioned between the mirror and themagnetic prism.

The multiple electron mirror arrangement can be used in a number ofways. A system having a plurality of electron mirrors will permit theconstruction of systems having higher image contrast and betterresolution since the energy spread of the electrons in a beam can bereduced at one or more electron mirrors by allowing the mirror tocollect high energy electrons while mirroring the low energy electronsfor direction to a second mirror. Energy filtering in this manner can beused to advantage in any device or system where high resolution isneeded. A multiple electron mirror arrangement also permits the transferof a mirror image from one mirror to another mirror which can be usefulin an information storage and retrieval system. The latter arrangementis used herein to illustrate a use of the invention.

BRIEF DESCRIPTION OF THE DRAWING The invention will be understood andits various advantages will become apparent from the description tofollow given in conjunction with the accompanying drawings wherein:

FIG. I is a schematic showing of the operation of an electron mirror;

FIG. 2 is a schematic showing of one embodiment of the invention;

FIG. 3 is a schematic showing of a second embodiment of the invention;

FIG. 4 is an end view of the magnetic prism 10 used in the embodimentshown in FIG. 3 and is taken along 4-4 of that figure; and

FIG. 5 is a top view of the preferred structure for the electronfocusing lenses used in the apparatus of FIG. 3.

DESCRIPTION In order that the invention presented herein can be bestunderstood, the known principles of electron mirror operation will besummarized using FIG. 1 of the drawing. FIG. 1 shows a simple electronmirror arrangement which includes an apertured plate 1 which ispositioned parallel to and spaced a few millimeters from the mirrorspecimen surface 2. The space between the apertured plate I and themirror specimen surface 2 is referred to as the mirror region. A strongelectric field is maintained in the mirror region by biasing theapertured plate a few kilovolts positive and the mirror specimen 2 a fewvolts negative relative to the electron source. The electric field canbe adjusted so the electrons fired into the mirror region do not havesufficient energy to reach the specimen surface 2 and consequentlyexecute the parabolic paths 3 shown in FIG. 1. If these parabolic pathsare unperturbed by the specimen 2, the cross-section of the mirroredbeam is uniform in density. However, differences in the surfacetopography, surface potential distribution or magnetization of thespecimen cause field gradients which produce perturbations of theparabolic paths. The effect is observed as spatial modulation of thedensity of the mirrored beam. This modulation can be observed as anelectron mirror image of the specimen by allowing the beam to impinge ona phosphorus screen.

The distance from the mirror specimen surface 2 to the point at whichthe electrons reverse their direction can be increased by making themirror bias more negative relative to the electron source. This will, ofcourse, cause the spatial modulation to decrease because the intensityof the field gradients caused by the specimen decreases with theincreasing distance from the mirror specimen surface 2. The mirror biastherefore provides a means for preventing any spatial modulation of theelectron beam. An electron mirror image can therefore be turned on oroff as needed by the bias on the mirror specimen relative to theelectron source.

Since the mirrored electrons do not touch the specimen 2, it is possibleto have non-destructive study of the nature of the specimen surface. Theelectrons have typically only a few electron volts of energy at thepoint of turn-around so they are very sensitive to differences in theelectric, magnetic and topographic properties of the specimen surface.This makes the electron mirrors more sensitive than other electron beamtechniques. This sensitivity and the non-destructive nature of electronmirrors make them very useful as memory devices.

One embodiment of the invention presented herein is disclosed inconnection with the multiple electron mirror memory apparatusschematically shown in FIG. 2 and described in greater detail incopending patent application Ser. No. 272,779, filed July 18, 1972, nowU.S. Pat. No. 3,789,370, having the same assignee as this application.Spaced about a magnetic prism 10 are a modulating electron mirror 4, twostorage mirrors 6 and 8, an electron gun 11 and a readout array 12.Three element electron focusing lenses 13, l4, l and 16 provided formirrors 4, 6 and 8 and the readout array 12, respectively, are alsopositioned around the prism 10. In addition to the focusing lens 14, themirror 6 has an objective lens 18 positioned near its storage surfaceand two deflection units 19 and 20 positioned between the objective lens18 and the focusing lens 14. An objective lens 21 and two deflectionunits 22 and 23 are similarly provided for mirror 8. The objectivelenses 18 and 2] may be three-element multi-apertured immersion typelenses. The components described in connection with FIG. 2 are securedwithin a housing represented by the dotted line surrounding thecomponents to permit operation of the components at a pressure ofapproximately Torr. For the sake of brevity and clarity, electricalconnections have not been shown in FIG. 2.

A general description will be given of the operation of the apparatuswhich is sufficient for an understanding of the invention. The apparatushas a write mode and a read mode of operation. The write mode providesfor the transfer of an entire page (plurality of bits) of informationfrom mirror 4 to either of mirrors 6 and 8. This write mode can bereferred to as a parallel write mode since all bits of information inthe page are transferred simultaneously. A page of information ispresented at the surface of the modulating mirror 4 in the form of apattern of charge determined by the information bits. Each page ofinformation transferred can be stored at a different one of a pluralityof storage areas at the mirror surface of mirrors 6 and 8 or can beapplied to a readout array 12. In addition, a transfer made to mirror 6can be repeated at mirror 8 to provide redundant storage to improvereliability of the system. The read mode of operation provides for thetransfer of the entire page of information present at a selected storagearea of mirrors 6 and 8 to readout array 12. This read mode can becalled a parallel read since all bits of information in the page aretransferred simultaneously. The read mode of operation provides forrandom access to the stored information in that the electron beam iscontrolled by the voltages applied to the deflection units 19, 20 and22, 23 for address of any storage area to be read.

During the write mode, a well collimated eletron beam generated by theelection gun I! is accelerated and directed at the center of themagnetic prism 10 which deflects the beam so as to pass through thefocusing lens 13 for modulating mirror 4. In the embodiment shown inFIG. 2, the beam is deflected 30. The focusing lens 13 is constructedwith focusing properties to cause the electron beam to present across-section large enough to encompass all of the modulation sitesmaking up the page of information at the surface of the mirror 4. Duringthe write mode the pattern of charge presented by the modulation siteson the surface of the mirror 4 spatially modulates the mirrored electronbeam into a data or information pattern corresponding to the pattern ofcharge presented by the modulation sites.

After being mirrored and modulated at the modulating mirror 4, theelectron beam passes back through the focusing lens 13 to the magneticprism 10 where it is again deflected 30 by the prism 10 for passagethrough the focusing lens 14 for the mirror 6. The lens 14 has focusingproperties causing the beam to be collimated and reduced incross-section. The beam then passes through the two deflection units 19and 20 which serve to shape the beam and direct it to the desiredlocation or address in the multi-apertured immersion objective lens 18.Assuming the information contained in the modulated electron beam is tobe stored at mirror 6, the bias on mirror 6 is set to allow the beam toimpinge on the addressed storage area on the surface of mirror 6 todeposit a pattern of charge corresponding to the beam modulationpattern. If the information is to be stored at mirror 8 instead ofmirror 6, the bias on mirror 6 is set so the electron beam is passivelymirrored without any spatial modulation by mirror 6. The beam passesback through the objective lens 18, the deflection units 19 and 20 andthe focusing lens 14 to the magnetic prism 10 where it is againdeflected 30 by the prism 10 for passage through the focusing lens 15for mirror 8. The beam then passes through the two deflection units 22and 23 where it is directed to the desired location or address in themulti-apertured immersion objective lens 21. Since the informationcontained in the modulated beam is to be stored at mirror 8, the bias onmirror 8 is set to allow the beam to impinge on the addressed storagearea on surface of mirror 8 to create a pattern of charge correspondingto the beam modulation pattern. For redundancy purposes, the page ofinformation presented at the modulation mirror 4 can be stored atmirrors 6 and 8. It is then possible during the read mode to read aselected page of information from each of the storage mirrors andpresent it to the readout array 12 to permit a comparison to be made ofthe information bits obtained from each page of information.

The information contained in the modulated beam can, if desired, bepassively mirrored by each of the mirrors 6 and 8 and applied via thefocusing lens 16 to the readout array 12. The focusing lens 16 hasfocusing properties causing the cross-section of the beam to beincreased so that the spacing between the modulation sites within themodulated electron'beam match the spacing between the elements of thereadout array 12. The readout array 12 simultaneously senses theelectrons in the beam representative all information sites contained inthe modulated beam.

When retrieval or reading of the stored information is desired, themodulating mirror 4 does not modulate the electron beam, but simplyserves as a passive element to maintain the beam path by merelymirroring the beam for passage to mirror 6. The deflection units 19 and20, in accordance with the voltages applied to units 19 and 20, directthe beam to the desired location or address in the multi-apertured lens18. Assuming the page of information at the addressed storage area ofmirror 6 is to be read or retrieved, the mirror 6 is biased so the beamis spatially modulated by the stored charge pattern at the addressedstorage area and mirrored for travel to mirror 8. The beam is directedby the deflection units 22 and 23 to the addressed location in themulti-apertured lens 21 and is passively mirrored by mirror 8. The beamis then presented to sensing units of the readout array 12 to providesimultaneous readout of all information sites contained in the modulatedbeam.

In the event the information to be retrieved is located at a storagearea of mirror 8, the electron beam is passively mirrored by theelectron mirror 6 and then modulated by the pattern of charge at theaddressed area of mirror 8 prior to being presented to the readout array12.

As indicated in connection with the general description regarding theoperation of electron mirrors, it is apparent that the voltagespresented at the modulating mirror 4 and storage mirrors 6 and 8determine whether the electron beam will be modulated and mirrored,whether it will impinge on the surface of the mirror or be passivelymirrored as is required for the write and read operations that have beendescribed. Details regarding the various voltages used for operation ofthe apparatus for the write, read and erase operations are set forth indetail in the copending application, now U.S. Pat. No. 3,789,370,referred to earlier as are details for construction of the variouscomponents and their assembly to form the apparatus illustrated in FIG.2.

FIG. 3 is another embodiment of the invention and differs from thatshown in FIG. 2 in that a different magnetic prism is used. The prism 10used in the embodiment of FIG. 3 is formed using two U-shaped typemagnets 61 and 62 which can be of the permanent or electromagnetic type.FIG. 4 is an end view of the prism 10 taken along 44 of FIG. 3. Themagnetic polarity shown is that which is required to have the electronbeam deflected as indicated by the dotted line path which begins at theelectron beam source 11. As is apparent, a prism using two U-shapedmagnets as described has a well confined magnetic path which in amultiple electron mirror system reduces the possibility of anyinterference with the electron beam that may be caused by a magneticfield which is not as well confined.

The reference numerals used in FIG. 2 to identify the various componentsin that embodiment are applied,

where applicable, to corresponding components used I in the apparatus ofFIG. 3. It should be noted, however, I

that the embodiment of FIG. 3 shows the use of more than two storagemirrors 6 and 8. The additional storage mirrors and their associateddeflection units, objective lenses and electron focusing lenses areidentified by the addition of a letter to the basic reference numeral.Thus, the additional storage mirrors positioned on the same side of theprism 10 as storage mirror 6 are identified with reference 6A and 6B.The objective lens for storage mirror 6A is identified by the referencenumeral 18A, while numerals 19A and 20A designate the deflection unitsfor storage mirror 6A. The electron beam focusing lens for storagemirror 6A is identified by the reference 14A. Similar designations aremade for storage mirrors 6B. In a similar manner, the additional storagemirrors positioned on the same side of the prism 10 as storage mirror 8are identified by references 8A and 88. References 15A, 21A, 22A and 23Aidentify the electron focusing lens, the objective lens and the twodeflection units, respectively, which are used with the storage mirror8A. The electron focusing lens, objective lens and two deflection unitsfor the storage mirror 88 are identified in a similar manner usingreferences 15B, 21B, 22B and 233, respectively.

Operation of the embodiment of FIG. 3 is substantially the same as thatset forth for the embodiment of FIG. 2 except for the number of storagemirrors used and the fact that the degree to which the electron beam isdeflected by the prism 10 is substantially less than that required forthe FIG. 2 structure. The linear configuration of the apparatus shown inFIG. 3 theoretically permits any number of electron mirrors to be usedwhich is not the case for the circular configuration of FIG. 2.

The electron focusing lenses 13,14, 15 and 16 for the FIG. 2 embodimentare formed using three concentric rings with holes of appropriate sizeprovided for passage of electron beam with the spacing between therings, the size of the holes and the voltage applied to the three ringsdetermining the focusing characteristic for the lens. These factors areset forth in detail in the copending application, now U.S. Pat. No.3,789,370, referred to at the beginning of this description.Threeelement type focusing lenses can also be used for the structure perFIG. 3. The linear configuration, however, permits the use of straightelements rather than ring shaped which greatly simplifies the machiningproblem. FIG. 5 is a top view of a three element arrangement for thefocusing lenses 14, 14A and 148, for use in the FIG. 3 embodiment. Asimilar arrangement can be used to provide focusing lenses 15, 15A and158.

A multiple electron mirror makes it possible to provide higher imagecontrast and better resolution, if desired, by reducing the energyspread of the electrons in the beam. This is accomplished by providingan electron mirror which is biased to collect high energy electronswhile mirroring the low energy electrons. In the case of the twoembodiments shown, the electron beam would be first directed to anelectron mirror which is biased to reduce the energy spread. The beamwould be mirrored and directed to a modulating electron mirror.

In the light of the above teachings, other alternative arrangements andtechniques embodying the invention will be suggested to those skilled inthe art. The scope of protection afforded the invention is not intendedto be limited to the specific embodiments disclosed, but

is to be determined only in accordance with the appended claims.

What is claimed is: 1. An electron beam system including a magneticprism; an electron beam source for providing an electron beam andpositioned to direct the electron beam to the magnetic prism; aplurality of electrically biased electron mirrors, with one positionedto initiallly receive the electron beam from the magnetic prism, saidone electron mirror biased to return the electron beam to the magneticprism and another of said plurality of electron mirrors positioned toreceive the electron beam returned to the magnetic prism by said oneelectron mirror. 2. An electron beam system is accordance with claim 1wherein said plurality of electron mirrors are positioned about saidmagnetic prism.

3. An electron beam system in accordance with claim 1 wherein saidmagnetic prism is positioned centrally of said plurality of electronmirrors.

4. An electron beam system in accordance with claim 2 wherein saidmagnetic prism includes a set of Helmholtz coils.

5. An electron beam system in accordance with claim 1 wherein saidmagnetic prism includes two magnets, each having two spaced poles ofopposite magnetic polarity, said magnets positioned with the poles ofone magnet opposite and magnetically opposed to the poles of the othermagnet.

6. An electron beam system in accordance with claim 5 wherein said twomagnets are U-shaped.

7. An electron beam system in accordance with claim 5 wherein saidmagnets are permanent magnets.

8. An electron beam system in accordance with claim 5 wherein saidmagnets are electromagnets.

9. An electron beam system in accordance with claim 5 wherein saidmagnetic prism has one side defined by one set of opposed poles of saidtwo magnets and a second side defined by the other set of opposed polesof said two magnets with at least one of said plurality of electronmirrors positioned on said one side of said magnetic prism and at leastanother one of plurality of electron mirrors positioned on said secondside of said magnetic prism.

10. An electron beam system in accordance with claim 1 wherein at leastone of said plurality of electron mirrors is biased to collect highenergy electrons and mirror the low energy electrons whereby the energyspread of the electron beam is reduced.

1. An electron beam system including a magnetic prism; an electron beamsource for providing an electron beam and positioned to direct theelectron beam to the magnetic prism; a plurality of electrically biasedelectron mirrors, with one positioned to initiallly receive the electronbeam from the magnetic prism, said one electron mirror biased to returnthe electron beam to the magnetic prism and another of said plurality ofelectron mirrors positioned to receive the electron beam returned to themagnetic prism by said one electron mirror.
 2. An electron beam systemis accordance with claim 1 wherein said plurality of electron mirrorsare positioned about said magnetic prism.
 3. An electron beam system inaccordance with claim 1 wherein said magnetic prism is positionedcentrally of said plurality of electron mirrors.
 4. An electron beamsystem in accordance with claim 2 wherein said magnetic prism includes aset of Helmholtz coils.
 5. An electron beam system in accordance withclaim 1 wherein said magnetic prism includes two magnets, each havingtwo spaced poles of opposite magnetic polarity, said magnets positionedwith the poles of one magnet opposite and magnetically opposed to thepoles of the other magnet.
 6. An electron beam system in accordance withclaim 5 wherein said two magnets are U-shaped.
 7. An electron beamsystem in accordance with claim 5 wherein said magnets are permanentmagnets.
 8. An electron beam system in accordance with claim 5 whereinsaid magnets are electromagnets.
 9. An electron beam system inaccordance with claim 5 wherein said magnetic prism has one side definedby one set of opposed poles of said two magnets and a second sidedefined by the other set of opposed poles of said two magnets with atleast one of said plurality of electron mirrors positioned on said oneside of said magnetic prism and at least another one of plurality ofelectron mirrors positioned on said second side of said magnetic prism.10. An electron beam system in accordance with claim 1 wherein at leastone of said plurality of electron mirrors is biased to collect highenergy electrons and mirror the low energy electrons whereby the energyspread of the electron beam is reduced.